A sign, for the moment
A flash of light. Zap! An instant of energy that was recorded in a tank of liquid xenon, a mile and a half underground in South Dakota, on the night of June 16, 2023. Three years later, the team that detected it is coming out to explain it to the world. “We have discovered dark matter,” they would say, if this were a work of science fiction. In reality, the researchers have come out to say: “For the moment, it is an observation that needs validation”.
The detector used is called LZ (LUX-ZEPLIN) and its mission is to hunt for dark matter. This invisible substance, which neither emits nor absorbs light, constitutes approximately 85% of all matter in the universe. We have known it exists since the thirties, when Fritz Zwicky calculated that galaxy clusters were rotating too fast for the visible matter they contained. In the seventies, Vera Rubin confirmed this by measuring the rotation curves of spiral galaxies. Ninety years later, we still do not know what it is.
A classic hypothesis is the WIMP, an English acronym for ‘weakly interacting massive particles’. They would be heavy particles, theoretically observable if they collide with an atomic nucleus. To detect these collisions, the LZ team filled a tank with ten tons of ultra-pure xenon, buried it in an old gold mine, and surrounded it with layers of shielding to filter out all cosmic noise. In two hundred and twenty days of observation, one flash stood out above the background: a nuclear recoil of 248 keV (a flying mosquito carries millions of times more kinetic energy), compatible with a WIMP of about 1,000 GeV, a thousand times heavier than a proton.
The probability studied by the researchers that this observation is real and significant does not yet reach the field's standards. Hence the long review of the data and the scientific team's caution. The confirmation, if it comes, will come from other experiments: XENON, the Italian National Laboratory of Gran Sasso, and PandaX, the Chinese particle detector in Jinping.
The team acknowledges another methodological weakness. Usually, to avoid bias, analysts inject fake events into their own data and do not identify them until the end: this is the salting technique, a way of forcing themselves to work blindly, similar to the administration of new drugs during clinical trials. This time, the simulated events did not cover the high-energy zone where the flash in question appeared. It does not invalidate the result, but it calls for more caution.
This flash, which has needed three years of scrutiny to reach the public, may still mean nothing. But the fact that we have to wait for its confirmation, and not accept it at first glance, is what separates knowledge from rumor. In a field where the object of study is the cosmological matter of the universe, it will not come down to a year sooner or a year later.